The present invention relates to power device drive circuits, and in particular, such drive circuits incorporating optocouplers. Even more particularly, the invention relates to circuits for eliminating false triggering of power devices due to dv/dt sensitivity of optocouplers used in the driver circuits. For example, the driver circuit may drive the gate or base of a power transistor circuit. The present invention can be applied to an optocoupler which drives the gate or base of a power transistor switching circuit, for example, a bipolar transistor, MOSFET or IGBT switching circuit. The present invention thus relates to a circuit for eliminating false triggering of power devices due to dv/dt or noise sensitivity of optocouplers in the gate drive circuit.
Optocouplers are often used to provide electrical isolation between the control circuit and the power transistor device in various applications. Problems arise when the voltage across the isolation barrier of the opto-isolator changes very rapidly. The output of the optocoupler can be erroneous because the receiver in the optocoupler cannot distinguish between the control signal and the noise signal through the stray capacitance between the transmitter of the optocoupler and the receiver of the optocoupler.
An optocoupler with appropriate control signals and biasing circuit is shown schematically in FIG. 1. FIG. 2 shows waveforms in the circuit of FIG. 1. In the optocoupler, a light emitting diode typically controls the isolated output. In FIG. 1, V0 is low when the LED is not energized and V0 is high when the LED is energized. As shown in FIG. 2, when the voltage V1 is 0, i.e., the switch is open, the output V0 of the optoisolator is commanded to zero. When the voltage B3, which represents the voltage across the isolation barrier of the opto-isolator, changes rapidly, the output of the optocoupler can be erroneous. When the optocoupler is used to control a power device, the dv/dt generated false output can destroy the power device. For example, when B3 changes rapidly, the output V0 of the optocoupler goes momentarily high as shown at 10 in FIG. 2. When V1 goes high, the output V0 of the opto-isolator is normally commanded high, as shown in FIG. 2. However, dv/dt generated by B3 can cause the output to go low momentarily, as shown at 20, which corresponds to the trailing edge of dv/dt. Also, shown in FIG. 2 is a negative going spike dv/dt 30 which can manifest itself as a corresponding spike 40 in the signal V0. Similarly, a positive going dv/dt spike 50 can manifest itself as a negative going spike 60 in the signal V0 at the output of the optocoupler.
FIG. 3 shows a typical prior art drive circuit incorporating optocouplers. The circuit includes a respective optocoupler 1, 2 for each power device 9A, 9B, a respective buffer stage 7A, 7B, respective coupling resistors 8A, 8B and recirculating diode 10A and 10B. The load 11, 12 is connected to output V7.
In this circuit, assuming transistor 9A is off, when V1B goes low (0V), transistor 9B turns off and the output impedance of the circuit increases to a large value because both transistors 9A and 9B are off. Optocoupler 2A is then subject to dv/dt stress if the output voltage V7 changes, which can only occur when both transistors 9A and 9B are off. During this time, optocoupler 2A can generate a false output and turn transistor 9A on and off, increasing switching losses and possibly destroying the device. At some later time, V1A is commanded high and transistor 9A turns on.
In the prior art drive circuit, which may be a bipolar, MOS or IGBT gate drive circuit, the erroneous output of the optocoupler can cause misfiring and destruction of the power devices.
The dv/dt sensitivity of the optocoupler itself can be changed only by changing the internal construction of the optocoupler, e.g. by reducing capacitances, adding shielding, etc. The invention seeks to eliminate the effect of erroneous optocoupler outputs.
The invention is based on the realization that in most applications, the occurrence by dv/dt stress is predictable, and by temporarily disabling the receiver output of the optocoupler or the electronic circuit coupled to the receiver output, the dv/dt generated noise spike can be rejected or eliminated from reaching the power device control electrode.